Read the following passage and choose the letter A, B, C, or D to indicate the correct answer to each of the questions. Question 21-23 In the future, people will build houses on the moon. The best place for houses in a moon town will be underground. Underground houses will be safe, and the temperatures will not be very high during daytime and not very low at night. Moon homes may be balloons with oxygen, and people will wear oxygen masks when they are away from home because the moon has no air around it. The house might have beds, tables, and chairs, so people will sleep, eat, and work in their home. Moon-house building will be easy. Question 21. In a moon town, what is the best place for house? * 4 điểm A. a balloon B. on the ground C. underground D. in the air Question 22. Why do people need oxygen when they stay on the moon? * 4 điểm A. Because the moon is too large B. Because the moon houses are full of air C. Because people sleep, eat, and work outside their houses D. Because the moon has no air.